There is a growing need for a range of positive-acting imaging materials able to complement negative-acting Dry Silver materials, particularly for application in graphic arts and medical imaging. For such uses, prospective imaging materials should desirably exhibit the following characteristics:
(i) contact (10.sup.5 to 10.sup.3 erg/cm.sup.2) or higher speeds, PA0 (ii) dry processing, preferably using standard Dry Silver processing conditions, and PA0 (iii) thermally stable for archival storage purposes.
The prior art has long sought to provide suitable positive-acting photothermographic materials, examples of which include those materials disclosed by British Patent Nos. 1156933, 1172425, 1507829, 2022277 and 2195463; European Patent Nos. 223587, 301539, 320020 and 362827; U.S. Pat. Nos. 3589901, 4075017, 4124387, 4587198, 4753862, 4761360, 4772541, 4800149, 4814252 and 4865942, and Japanese Patent Nos. 53-120520, 57-089750, 57-101832, 58-040543, 58-040544, 60-030931, 61-107243, 61-183460, 61-188535, 61-022841, 62-187837, 62-178742, 63-034536 and 63-330064.
Our copending British Patent Application No. 9121789.3 of even date discloses positive-acting photothermographic elements having a photosensitive medium comprising a dispersion of a reducible silver source and a reducing system for silver ion and a photocurable composition. The photocurable composition comprises a free radical curable resin and a photoinitiator having an absorbance to radiation in the wavelength range of 340 to 440nm. In those areas of the element exposed to radiation, the photoinitiator promotes curing of the free radical curable resin, thereby increasing the glass-transition temperature of the resin in those regions, and effectively immobilising or otherwise preventing the reducing system from reacting with the reducible silver source during subsequent thermal processing. Examples of suitable photoinitiators include onium salts, such as iodonium and sulphonium salts, which may be used alone or in combination with a sensitiser, e.g., oxonol dyes, 1,4-dihydropyridines and triarylpyrazolines.
The present invention seeks to provide alternative positive-acting photothermographic materials.